The Arby's Classic is defined by what it deliberately leaves out: there is no jus, no melted cheese, and no hard relish. The sliced-roast-beef sandwich usually depends on a wet finish, a roll dipped in beef stock or piled under cheese sauce, to keep a lean pile of meat from reading as dry. This build refuses that and stakes everything on the slicing and a thin sauce. Slow-roasted beef is shaved into a high, loose mound on a toasted sesame bun, and the only seasoning of consequence is Arby's Sauce, a thin, tangy, faintly sweet condiment that does the acid-and-moisture job a jus or a pickle does elsewhere. The defining decision is restraint: the sandwich is the beef and the slice, with almost nothing asked to carry it.
The craft is in the shave and the structure that restraint demands. The beef is roasted and then sliced extremely thin and piled loose rather than packed, because air in the mound is what keeps shaved beef tender in the hand; pressed tight or sliced thick it goes to rope. With no jus to lubricate it and no cheese to bind it, the slicing has to do all the work the wet finishes do in a French dip or an Italian beef. The bun is the structural counterweight: a sesame-seed bun, toasted so the crumb firms into a surface that braces a tall, loose pile without compressing to paste, since there is no soaked-roll trick holding this together. Arby's Sauce is applied thin on purpose. It supplies tang and a little sweetness to cut the salt of the beef and a trace of moisture to a dry build, but too much would flood a bun that has no jus-soaking job and is not engineered for it. The sandwich is assembled and eaten promptly, while the toast still has spine and the beef is still warm.
The variations are mostly a matter of scale and sauce. A larger build stacks more shaved beef on the same bun and tests how much loose meat the toasted crumb can brace. Horsey Sauce, a creamy horseradish condiment, swaps the thin tang for heat and fat. The Beef 'n Cheddar is the same shaved beef taken in the opposite direction, toward cheese sauce and a sweet dressing. Each of those is its own preparation and deserves its own article rather than being crowded in here.